Bile pigments in algae and mammals are synthesized from heme via a Two-Molecule Mechanism in which the lactam oxygen atoms on the external rings arise from molecular oxgen, but from different oxygen molecules. The events which occur in this process will be examined by measuring the mechanism of 18,1802 incorporation into bile pigment in red and blue-green algae. Bile pigment-apoprotein complexes (phycobiliproteins) associate into high molecular weight multimers in vivo to produce functional aggregates called phycobilisomes. The primary structure all allophycocyanin and phycocyanin (phycobiliproteins) to which bile pigment is covalently attached is being determined by automated sequence analysis. An attempt will be made to translate the mRNA's for the apoprotein moiety of allophycocyanin and phycocyanin to determine some of the events involved in regulation of phycobiliproteins synthesis in vivo.